Huntington's disease (HD) is caused by expansion of a polymorphic CAG repeat within exon 1 of huntingtin, a gene of unknwon function. In HD and mouse and cell culture models, huntingtin fragments containing the polyglutamine tract undergo progressive nuclear aggregation. We have established a Drosophila model of HD that shares important features of the human phenotype, including age- and repeat-length-dependent neuronal degeneration and death, as well progressive nuclear localization of expanded repeat-containing protein. The similarities between the polyglutamine-expanded phenotype in humans and flies suggest that the molecular mechanisms underlying polyglutamine-induced cell death are, at least in part, conserved from Drosophila to man. The long-term objective of the proposed research is to utilize our Drosophila model of HD to unravel the molecular mechanisms of polyglutamine-induced cell death in an effort to identify therapeutic targets. A three-pronged approach will be used: 1. The role of nuclear aggregation of polygluytamine-containing protein on cytotoxicity in vivo will be assessed by examining the distribution of various epitope-tagged constructs over time and by expressing these constructs in the presence of a nuclear export signal. We will also examine the interaction of polyglutamine-expanded fragments with both pathologic and wild type repeat lengths presented as truncated fragments or within the full length protein. 2. The role of identified genes in Drosophila that may modify polyglutamine-induced neuronal cell death will be examined by expressing the Q120 transgene in a background homozygous for mutations in these genes. We will assess potential disease-modifying genes by expressing the Q120 construct in a genetic mosaic background, including patches homozygous for the mutation of interest. 3. A large-scale genetic screen for mutations that alter the photoreceptor degeneration associated with Q120 expression will be used to identify enhancers and suppressors of polyglutamine-induced cell death. Mutagenized males will be crossed to Q120-bearing females. Mutations affecting degeneration will be scored by examining the pseudopupil pattern and by scoring reversion to the wild type response in a UV choice test. Mutations will be localized and cloned using the technizue of "local hopping." Suppression or enhancement will be verified by co-expressing such mutations with Q120 lines and in inducible cell culture systems.